The present invention relates to an electrical band-pass filter and more particularly, to a piezoelectric filter of ladder type for use in electrical and electronic equipment which is constituted by a multi-mode resonator and a single resonator provided on a piezoelectric substrate.
Conventionally, there has been known a piezoelectric filter of ladder type having a construction, for example, as shown in FIG. 1 which includes a housing 1 molded with electrically insulative resin material to have a generally U-shaped cross-section, two-terminal type resonators C1 and C2 accommodated in the interior of the housing 1 together with corresponding terminal plates 2a and 2b and 3a and 3b. The resonator C1 is held between contact protuberances 4a and 4b respectively protruding in semi-spherical shape from central portions of the terminal plates 2a and 2b, and the resonator C2 is held between contact protuberances 5a and 5b also protruding in semi-spherical shape from central portions of the terminal plates 3a and 3b. Lead terminals 6a and 6b and 7a and 7b are respectively provided to extend through side walls 1a and 1b of the housing 1, with the lead terminal 6a being connected to the terminal plate 2a and the lead terminal 7a to the terminal plates 2b and 3a, while the lead terminals 6b and 7b are connected to the terminal plate 3b. The two-terminal type resonators C1 and C2 respectively function as a series resonance element and a parallel resonance element for constituting a ladder type filter as shown in FIG. 2.
However, the known ladder type filter as described above has several disadvantages. Since the terminal plates 2a, 2b, 3a and 3b are required for holding the two-terminal type resonators C1 and C2 therebetween, the number of parts involved is undesirably increased with consequent complication of the work of assembling the filter. Moreover due to the arrangement to hold the resonators C1 and C2 between the semi-spherical contact protuberances 4a and 4b and 5a and 5b respectively, the resistance of the filter against shocks is reduced, while resonators utilizing thickness vibration cannot be employed owing to the specific construction.
For overcoming the disadvantages as described above, there has also been conventionally disclosed, for example, in Japanese Patent Publication Tokkyosho No. 39-8251, an electrical filter as shown in FIG. 3. The filter of FIG. 3 includes a comparatively thin plate or substrate 12 of piezoelectric material, at least two electrodes 18 and 20 provided in spaced relation from each other on one main surface 14 of the substrate 12, and corresponding electrodes 22 and 24 provided on the other main surface 16 of the substrate 12, with the electrodes 18 and 22 and 20 and 24 facing each other to respectively constitute electrode pairs. The total area of the electrodes 18 and 20 on the one main surface 14 is generally smaller than the area of that main surface 14, and each of the above electrode pairs functions together with the piezoelectric material held therebetween so as to constitute an electrical filter piezoelectric circuit element having resonators 26 and 28 independently vibrating in the same thickness mode at different fundamental resonance frequencies, and thus, the terminal plates 2a, 2b, 3a, 3b, etc. described as employed in FIG. 1 are omitted to provide the electrical filter of simple construction.
Incidentlly, in the case where the ladder type filter is to be constituted by the resonators 26 and 28 as described above, it is necessary to raise the resonance frequency fr1 of the series resonance element 26 higher than the resonance frequency fr2 of the parallel resonance element 28 of the ladder type filter, as is seen from FIG. 4, but due to the fact that both of the resonators 26 and 28 are formed on the single substrate 12 of piezoelectric material for vibration at the same thickness vibrating mode, it is difficult to raise the above resonance frequency fr1 higher than the resonance frequency fr2 to any large extent, with the result that the bandwidth of the ladder type filter is reduced.